Just an update: after keeping the yellow tang in the separate 10G for about a week, the blue tang started looking more at home in the main tank. So I returned the yellow before the blue establishes teritory. I guess I added him to soon, because he went back to picking on the blue. He only let him swim in 1/4 of the tank without bothering him. as soon as he gets in the other 3/4 he will chase him out. I ended up rearanging the rocks a little and within a day or two they were OK. The blue is swiming all over the tank now and the yellow hardly seems bothered.